Cutting inserts employed for metal machining are commonly fabricated from composite materials due to their attractive combinations of mechanical properties such as strength, toughness, and wear resistance compared to other tool materials such as tool steels and ceramics. Conventional cutting inserts made from composite materials, such as cemented carbides, are based on a “monolithic” construction, i.e., they are fabricated from a single grade of cemented carbide. In this manner, conventional monolithic cutting tools have the same mechanical and chemical properties at all locations throughout the tool.
Cemented carbides materials comprise at least two phases: at least one hard ceramic component and a softer matrix of metallic binder. The hard ceramic component may be, for example, carbides of any carbide forming element, such as titanium, chromium, vanadium, zirconium, hafnium, molybdenum, tantalum, tungsten, and niobium. A common example is tungsten carbide. The binder may be a metal or metal alloy, typically cobalt, nickel, iron or alloys of these metals. The binder “cements” the ceramic component within a matrix interconnected in three dimensions. Cemented carbides may be fabricated by consolidating a powdered metal of at least one powdered ceramic component and at least one powdered binder.
The physical and chemical properties of cemented carbide materials depend in part on the individual components of the metallurgical powders used to produce the material. The properties of the cemented carbide materials are determined by, for example, the chemical composition of the ceramic component, the particle size of the ceramic component, the chemical composition of the binder, and the ratio of binder to ceramic component. By varying the components of the metallurgical powder, tools, such as inserts, including indexable inserts, drills and end mills can be produced with unique properties matched to specific applications.
In applications of machining today's modern metal materials, enriched grades of carbide materials are often desired to achieve the desired quality and productivity requirements. However, cutting inserts fabricated from a monolithic carbide construction using the higher grades of cemented carbides are expensive to fabricate, primarily due to the high material costs. In addition, it is difficult to optimize the composition of the conventional monolithic indexable cutting inserts comprising a single grade of carbide material to meet the different demands of each location in the insert.
Composite rotary tools made of two or more different carbide materials or grades are described in U.S. Pat. No. 6,511,265. At this time, composite carbide cutting inserts are more difficult to manufacture than rotary cutting tools. First, the size of cutting inserts are, typically, much smaller than rotary cutting tools; second, the geometry, in particular cutting edges and chip breaker configurations of today's cutting inserts are complex in nature; and third, a higher dimensional accuracy and better surface quality are required. With cutting inserts, the final product is produced by pressing and sintering product and does not include subsequent grinding operations.
U.S. Pat. No. 4,389,952 issued in 1983 presents an innovative idea to make composite cemented carbide tool by first manufacturing a slurry containing a mixture of carbide powder and a liquid vehicle, then creating a layer of the mixture to the green compact of another different carbide through either painting or spraying. Such a composite carbide tool has distinct mechanical properties between the core region and the surface layer. The claimed applications of this method include rock drilling tools, mining tools and indexable cutting inserts for metal machining. However, the slurry-based method can only be applicable to indexable cutting inserts without chip breaker geometry or the chip breaker with very simple geometry. This is because a thick layer of slurry will obviously alter the chip breaker geometry, in particular widely used indexable cutting inserts have intricate chip breaker geometry required to meet the ever-increasing demands for machining a variety of work materials. In addition, the slurry-based method involves a considerable increase in manufacturing operations and production equipment.
For cutting inserts in rotary tool applications, the primary function of the central region is to initially penetrate the work piece and remove most of the material as the hole is being formed, while the primary purpose of the periphery region of the cutting insert is to enlarge and finish the hole. During the cutting process, the cutting speed varies significantly from a center region of the insert to the insert's outer periphery region. The cutting speeds of an inner region, an intermediate region, and a periphery region of an insert are all different and therefore experience different stresses and forms of wear. Obviously, the cutting speeds increase as the distance from the axis of rotation of the tool increases. As such, inserts in rotary cutting tools comprising a monolithic construction are inherently limited in their performance and range of applications.
Drilling inserts and other rotary tools having a monolithic construction will, therefore, not experience uniform wear and/or chipping and cracking at different points ranging from the center to the outside edge of the tool's cutting surface. Also, in drilling casehardened materials, the chisel edge is typically used to penetrate the case, while the remainder of the drill body removes material from the casehardened material's softer core. Therefore, the chisel edge of conventional drilling inserts of monolithic construction used in that application will wear at a much faster rate than the remainder of the cutting edge, resulting in a relatively short service life. In both instances, because of the monolithic construction of conventional cemented carbide drilling inserts, frequent tool changes result in excessive downtime for the machine tool that is being used.
There is a need to develop cutting inserts, optionally comprising modern chip breaker geometry, for metal machining applications and the methods of forming such inserts.